1. Field of the Invention
The present invention relates to a red-emitting phosphor
2. Background Art
Liquid crystal displays have been popularly used as color image displaying devices in household TV sets and in information indicating instruments. A color liquid crystal display normally comprises a backlight which is a cool cathode fluorescent lamp or a light-emitting diode (hereinafter, referred to as “LED”) emitting light containing red, blue and green components. The display also comprises a color filter that transmits the light components so selectively that each may corresponds to each of red, blue and green pixels, and the transmittance of the light penetrating through each color pixel is controlled by a liquid crystal element so as to produce a color gradation image. This displaying method is widely adopted at present. In the method, however, since white light is separated through the color filter, light in the wavelength range other than the displayed colors is shaded and hence not utilized. This means that energy of the light is wasted in the wavelength range other than the displayed colors, and therefore the above method is not of high energy use efficiency based on the total of light energy emitted from the LED.
In view of that, a new displaying method is proposed which uses plural phosphors capable of giving off luminescence of different colors. In the proposed method, each of the phosphors is provided on the place corresponding to the pixel of each luminescent color. Since each phosphor converts excitation energy into luminescence of the color similar to each pixel color, this method can improve efficiency both in energy use and in luminescence. JP-A 2003-255320 (KOKAI) discloses a liquid crystal display adopting this displaying method. The disclosed display comprises a fluorescent tube capable of emitting UV and blue light as an excitation source, and a color filter containing phosphors dispersed therein. Each of the phosphors emits luminescence of the color corresponding to the color displayed by each pixel. The publication also discloses examples of the phosphors usable in the display. However, although it is disclosed that UV and blue light is used as an excitation source, the above publication is silent about the advantage of near UV LED excitation light. JP-A 2007-25621 (KOKAI) discloses a color display comprising a blue excitation light source and phosphors capable of emitting red and green luminescence under the excitation with the blue light. In the disclosed display, the phosphors emitting red and green luminescence are used as red and green pixels, respectively. However, since the blue excitation light considerably leaks out from the red and green pixels, the display still needs to comprise a color filter in combination with the phosphors in order to improve the purity of the displayed colors. Consequently, the display adopting this displaying method inevitably has a complicated structure. This is a problem to improve. For the purpose of overcoming the problem, Yada et al.
disclose a light-emitting type liquid crystal display comprising a near UV LED as an excitation source (see, Yada et al., Synopses of 56th symposium of applied physics and related societies, 31p-p11-20, pp. 1489 (2009), [in Japanese]). This liquid crystal display can show satisfying color images without a color filter even if the excitation light leaks out. That is because visual sensitivity to the near UV excitation light is too low to impair the color purity of displayed images. However, Yada et al. disclose the above display only conceptually, and actually they do not concretely show phosphors suitable for this displaying method. In other words, they are silent about phosphors capable of emitting luminescence under the excitation with the near UV light.
Recently, in the field of research on displays employing phosphors, researchers have paid their attention to near UV LEDs as excitation light sources emitting light at approx. 400 nm. However, when excited by the near UV LEDs, conventional red-emitting phosphors give off luminescence of unclear color with insufficient intensity. Accordingly, there is room for improvement in conventional red-emitting phosphors.